Assessment of the Development Potential of Shallow Geothermal Energy Heating and Cooling Projects in Southern China Based on Whole-Lifecycle Methodology

Abstract

The development of shallow geothermal energy projects in southern China can meet the demand for regional heating and cooling energy and carbon emission reduction. However, research on constructing evaluation models for the development potential of shallow geothermal energy projects needs to be expanded. Therefore, this study adopted a hierarchical analysis method to construct a project development potential evaluation model based on the four aspects of resource endowment, economic evaluation, environmental impact, and social support for the shallow geothermal energy heating and cooling project (vertical buried pipe heat exchange system) in southern China and carried out case application and evaluation verification. The results of the study show that: (a) the weights of the four primary indicators for evaluating the development potential of shallow geothermal energy projects in southern China were resource endowment (0.3960) > economic evaluation (0.2847) > social support (0.1725) > environmental impact (0.1468); (b) four secondary indicators, namely heat exchange performance, incentive and supportive policies, geotechnical and thermal-physical parameters, and groundwater conditions, were more important; (c) the case evaluation score was 6.2911, and case application and evaluation verification were carried out. For projects with good potential for investment, contrary to the single financial NPV index evaluation results, our results are more in line with the actual operation results of the project. Thus, this evaluation system can provide a more comprehensive reference for shallow geothermal energy development and investment decision-making.

1. Introduction

Shallow geothermal energy, a renewable and clean energy source widely embedded in the ground, is mainly used to heat and cool buildings, and its utilization can reduce building energy use and carbon emissions [1]. Shallow geothermal energy is also considered to be the most effective decarbonization measure for heating and cooling buildings, and shallow geothermal energy systems are a sustainable alternative solution for heating and cooling buildings and for infrastructure in many parts of the world [2]. The operational energy consumption of China’s buildings, mainly heating and cooling energy consumption, accounts for 21.9% of the country’s total energy consumption, and their carbon emissions account for 21.6% of the country’s total energy-related carbon emissions, most of which come from the combustion of fossil energy [3]. As China’s economic development, people’s lives, and urbanization increase, China’s building operations, especially building heating and cooling energy consumption and carbon emissions, will further increase. China, especially in the southern region, will face the double pressure of securing energy and reducing carbon emissions at the same time [4]. Therefore, using and increasing the proportion of shallow geothermal energy to replace fossil energy for building heating and cooling will help China simultaneously meet the demands for energy supply growth and carbon emission reduction.

The annual growth rate of shallow geothermal energy for heating and cooling in China has exceeded 30% since 2004 and reached 8.0 billion square meters by the end of 2021, but it is mainly concentrated in the northern region, around the Bohai Sea [5]. Benefiting from the energy efficiency of centralized heating and the high proportion of clean energy use, such as shallow geothermal energy, northern China has a significantly lower average annual growth rate in building energy consumption and carbon emissions than southern China [6]. The demand for building energy in southern China is growing faster than that in the north, especially as the increase in extreme weather has led to a growing demand and call for centralized heating in the south [7]. Moreover, unlike the north, buildings in southern China need to take care of heating and cooling at the same time, and the utilization efficiency of shallow geothermal energy is higher, which is more suitable for shallow geothermal energy development [8]. However, the development of shallow geothermal energy for heating and cooling in southern China faces numerous problems. In addition to the difficulties of the low degree of exploration and evaluation, imperfect regulation and incentive support policies measures, and insufficient market players proposed by the shallow geothermal development plans of various provinces and cities [9,10], there is also the uncertainty of construction costs due to the influence of factors such as the complex geology of the southern region, as well as uncertainty in the market demand because of the lack of centralized heating [11]. In general, the development of the heating market in southern China is still very recent, and the lack of accurate knowledge about and scientific judgment of shallow geothermal energy and its value, whether by policymakers, investment enterprises, or the general public, has greatly restricted the development of shallow geothermal energy. Therefore, there is an urgent need for methods and models that can assess the potential value of shallow geothermal energy heating and cooling to assist decision-making regarding shallow geothermal energy projects and avoid missing good investment projects.

Accurate and detailed investigation and evaluation are the premise of shallow geothermal energy development and utilization, and are also the focus of domestic and foreign research on the development potential of shallow geothermal energy. In Europe, where shallow geothermal energy is more developed, there has been evaluation not only of geothermal potential in Europe as a whole [12], but also the research into the shallow geothermal potential for different countries and regions [13,14]. In China, scholars have investigated overall resource use throughout the whole country [15], as well as more detailed exploration of cities in different regions. For example, Wuhan evaluated the potential of shallow geothermal energy with different heat pump heat exchange systems [16], Changchun defined suitability zones for ground source heat pump technology according to geological and hydrological conditions [17], and Ludong and Guizhou carried out suitability zoning for ground source heat pump technology based on regional conditions [18,19]. These studies have provided important information for the development and utilization of shallow geothermal and the management of geothermal resources. However, these studies mainly focused on assessment of shallow geothermal resource potential. With the further development of shallow geothermal heating and cooling, comprehensive assessment of its economic value, thermal environmental impact, and social value has also become a research focus that cannot be ignored [20]. Economic assessment mainly examines the costs, benefits, and feasibility of geothermal projects to determine whether they are competitive or not [21]. Environmental benefit assessment, on the other hand, examines the non-market monetized environmental value of shallow geothermal systems and conducts an assessment of environmental burdens and benefits [22]. The social value of shallow geothermal energy is assessed in terms of the availability of incentive support policies in the study area, as well as public support and technological advancement.

Currently, academic research on the development potential of shallow geothermal energy is ongoing and is being analyzed and evaluated using different methodologies. In Madrid, in a study of 50 buildings using ground source heat pump systems, it was found that the use of geothermal systems for district heating and cooling could result in savings of 64% in primary energy and a reduction of 76% in CO₂ emissions [23]. D’Agostino et al. compared buildings using ground-source heat pumps with those dominated by gas boilers, and the results showed that buildings using ground-source heat pumps had a very low primary energy demand, saved a lot of money, and reduced CO₂ emissions [24]. Many scholars at home and abroad have used the hierarchical analysis method to analyze the suitability of shallow geothermal energy. Dong et al. used the hierarchical analysis method to measure the applicability range of ground source heat pump systems [25]. Zhang et al. used hierarchical analysis to determine the weight value of thermal conductivity, used the thermal conductivity to calculate heat transfer power, and measured the thermophysical parameters of the geotechnical layer to optimize heat pump system and reduce costs [26]. Liu et al. used hierarchical analysis to quantify a deep geothermal resource exploration and development zone delineation index and used a quantitative zoning evaluation method to delineate deep geothermal resource exploration and development prospect target zones on a regional scale [27]. Although there have been many studies on geothermal energy using hierarchical analysis, most of the above studies have concentrated on the detailed technical aspects of geothermal energy or on the micro level. However, there is a lack of studies on the systematic construction of an assessment system for shallow geothermal energy, which is suitable for the characteristics of southern China.

In terms of the tools used to assess the development potential of shallow geothermal energy projects, foreign scholars have developed, for example, GIS-based multi-criteria decision analysis combined with hierarchical analysis [28], while domestic scholars have used index methods such as hierarchical analysis [23] and fuzzy mathematical comprehensive evaluation [22], and have also used net present value [29] and real options [30] to evaluate the benefits of projects. Existing studies have shown that shallow geothermal energy development and utilization evaluation has a multidimensional perspective and many influencing factors; however, few scholars have been able to integrate these evaluation dimensions and factors to form a more comprehensive and systematic evaluation model. A single evaluation dimension will limit the comprehensiveness of the analysis, and the results of feasibility studies of shallow geothermal projects are not just the sum of various factors [31].

In summary, this study intended to identify all the factors affecting the development potential of the project throughout the whole lifecycle of shallow geothermal energy, optimize and adjust indicators through expert consultation, and construct an evaluation system of indicators of the development potential of shallow geothermal energy heating and cooling projects in southern China based on a hierarchical analysis method involving four dimensions––resource endowment, economic evaluation, environmental impact, and social support––as well as to establish quantitative evaluation criteria. The evaluation system was then validated through case studies, and a comprehensive quantitative evaluation system for the development potential of shallow geothermal heating and cooling projects in southern China was finally obtained.

2. Methodology

The analytic hierarchy process (AHP) is a method that can combine qualitative and quantitative methods to systematically and hierarchically analyze target influencing factors, and ultimately effectively deal with complex decision-making problems; it has been widely used in the field of shallow geothermal energy to evaluate the suitability of development [23,32]. In this study, the hierarchical analysis method was used to construct an evaluation system for the development potential of shallow geothermal energy heating and cooling in southern China. In addition to screening and identifying key indicators to construct a clear hierarchical evaluation system, evaluation factors were selected from representative and quantifiable perspectives to construct a reasonable scoring standard for the project. Finally, after calculating the weights of the indicators, scores were assigned according to the actual project parameters, and suggestions were made for the project’s investment decision-making.

In academic research, support for the generalizability of the AHP model to a single case is often questioned. However, studies in the field of geothermal energy have shown that the AHP method has the potential for cross-scenario application. Fuzzy AHP was combined with fuzzy comprehensive analysis to construct a model for evaluating the suitability of groundwater source heat pumps, which can be applied to the evaluation of geothermal resource development in different regions after adjusting the evaluation factors [25]. In addition, there is also a study to select seven key factors, the use of AHP to complete the four-level zoning of geothermal development in Henan Province, the construction of the evaluation system can be flexibly adjusted according to the characteristics of the resources and policies of each region, used in other regional geothermal planning [33]. The combination of AHP and Maxent to locate potential geothermal areas is also feasible in geothermal exploration under different geological conditions with parameter adjustment [34]. This study focused on successful scenarios to validate the core mechanisms of the AHP model and included failure analyses and comparisons with other assessment tools to further expand the application boundaries of the model.

2.1. Constructing a Hierarchical Structure

A reasonable evaluation system needs to be constructed hierarchically, so it is necessary to first identify and sort out the key influencing factors affecting the development of shallow geothermal energy heating and cooling and then filter and optimize the influencing factors after classifying and summarizing them, to finally create a representative and effective evaluation index system.

2.1.1. Analysis of Influencing Factors

Shallow geothermal development and utilization is mainly divided into the exploration, construction, and operation stages, and the whole lifecycle is characterized by a variety of geological, technological, policy, and market conditions of uncertain risk, which affects the assessment of its development value [35]. Currently, there are more studies on the development and assessment of shallow geothermal energy. In this study we read, analyzed, and organized the literature related to shallow geothermal energy regarding the four dimensions of resource endowment, economic evaluation, environmental impact, and social support and classified and summarized the factors that affect the development and utilization of shallow geothermal energy.

(i) Resource endowment

Evaluation of the development suitability of shallow geothermal energy resources is the main research direction of the current assessment of the development potential of shallow geothermal energy, and the results of the evaluation will determine whether and how the shallow geothermal energy project can be further carried out. The use of a vertical buried pipe soil source heat exchanger system is limited by the geographical area, so it is necessary to know the parameters such as the internal heat carrying capacity, the characteristics of the ground temperature field, the heat conduction coefficient of the geotechnical body, the heat exchange per unit of extension meter, etc.; furthermore, the design of the soil source heat exchanger system is based on the geothermal geologic conditions of shallow geothermal resources [25,36]. According to the relevant industry and regional standards for shallow geothermal exploration and evaluation in China, exploration of regional shallow geothermal energy should also fully investigate regional geological and hydrological conditions, and the investigation should include the lithological structure of the geotechnical layer, the distribution and burial conditions of aquifers, the thermal conductivity and specific heat capacity of the geotechnical body, and other thermal physical and geotechnical parameters, such as porosity, water content, and other physical parameters, as well as the natural distribution of the geothermal temperature field, the thermal response law, etc. [37]. For the assessment of the resource potential of shallow geothermal energy in foreign countries, geophysical or geochemical techniques are also used to measure the temperature, flow, and distribution of geothermal resources to analyze the geothermal potential [38]. The geothermal resource potential can also be predicted by simulating the distribution of geothermal resources and determining the depth of the heat source, temperature distribution, sustainability, and performance of the geothermal system through numerical and geological models [39]. Shallow geothermal suitability potential is also analyzed by comparing hydrogeological, geotechnical, geothermal, and geoenvironmental proximity coefficients [40]. Some relatively new approaches such as the infinite borehole field model assess the potential of geothermal resources by considering the thermal-geological details of geological formations, including downward increasing ground temperature, geothermal heat flux, thermal conductivity, heat capacity, porosity, density, and convective heat transfer [41]. Therefore, evaluation of shallow geothermal resource endowment mainly focuses on evaluation of project resource conditions such as geological conditions, groundwater conditions, geothermal physical parameters, and other project resource conditions in the project area.

(ii) Economic evaluation

Economic evaluation is an indispensable part of shallow geothermal energy development and utilization and project investment decision-making, mainly for investment estimation, operation cost estimation, revenue estimation, and benefit assessment. Foreign research on the economic analysis of shallow geothermal energy development is also mainly focused on evaluating the cost, benefit, and feasibility of geothermal projects to determine whether they are competitive or not [26]. Studies have shown that project scale, business model (length of concession period), heating and cooling prices, and operating costs of shallow geothermal energy heating and cooling projects are all important factors influencing development potential [42,43]. Globally, high investment costs and financing difficulties are common problems in the development of shallow geothermal energy heating and cooling, so the source of funding for the project, as well as high and low interest rates, will directly affect the development potential [44]. Considering the different heating market conditions in southern China, the number of users who choose to use shallow geothermal heating and cooling and the retention rate will also have a direct impact on project revenue.

(iii) Environmental impact

The environmental impact assessment mainly evaluates and predicts the ecological and environmental effects and environmental and geological problems that may be brought about by the development of shallow geothermal energy, and also includes positive and negative environmental effects. Specific assessment includes the impact of shallow geothermal energy utilization on the atmospheric environment, the impact of a groundwater heat exchange system on shallow groundwater, the impact of the chemical composition of the discharged fluid on the groundwater environment; whether it can produce ground subsidence, karst subsidence, and geocracks; and the impact of the leakage of circulating water from the soil–source heat exchange system on the quality of the groundwater. In particular, it is necessary to evaluate the impact of the development of shallow geothermal energy on the shallow geothermal field equilibrium, among other factors [25,45]. Overseas countries usually use full lifecycle assessment to examine the environmental burdens and benefits associated with the application of shallow geothermal systems, where the environmental burdens include the risk of geological subsidence and chemical contamination of soils caused by the development of shallow geothermal energy, and the benefits are mainly energy saving and emission reduction [1]. In addition, scholars have considered the impact of climatic and meteorological factors on the development of shallow geothermal energy, such as the impact of the heating and cooling season duration factor, and related studies have been performed to determine shallow geothermal energy potential by changing the duration of the heating and cooling seasons and the simulated lifespan [46]. However, spatial constraints and thermal disturbances from neighboring geothermal energy centers must also be taken into account when assessing the geothermal energy potential in urban areas; otherwise, the geothermal energy potential may be overestimated [47]. Therefore, assessment of the development potential of shallow geothermal energy projects must consider factors related to the environmental impact dimension.

(iv) Social support

For the development of shallow geothermal energy, in addition to the resource itself and the economic potential for development, support from the government, the public, and the technological development of related industries are also crucial. First of all, compared with solar and wind energy and other renewable clean energy resources known to the public, shallow geothermal energy is a low priority, and therefore receives less attention and fewer support subsidies. While the development of shallow geothermal energy also requires a huge investment, it is even more difficult, and therefore in the early stages of the development there is a need for supportive governmental policies, clear regulations, and standards and incentives [44]. In terms of public support, from a regional point of view, southern China is not a traditional centralized heating area; residents mainly use household or district-centralized heating and have a higher degree of autonomy in terms of choice of heating and cooling methods. The current public awareness of shallow geothermal energy and consumer confidence are insufficient, which directly affects users’ decision to choose shallow geothermal energy for centralized heating and cooling, making the benefits of shallow geothermal heating and cooling in southern China more uncertain; thus, these benefits need to be accurately assessed [48,49]. In addition to government and public support, technical support from shallow geothermal energy–related industries is also an important factor affecting the potential of shallow geothermal energy development. In addition to considering the reliability, efficiency, and safety of existing technologies, the popularity of technologies, the availability of maintenance services, and technical factors, domestic and foreign scholars have also focused on the impact of the decline in investment costs brought about by technological advances on the potential of shallow geothermal development and the value of investment [43].

After analyzing the factors influencing the four evaluation dimensions, a preliminary summary of evaluation indicators was obtained, as shown in

Table 1. Summary of factors influencing the development potential of shallow geothermal energy.

Dimension of Influence	Factors
Resource endowment	Geothermal bearing capacity, geothermal field equilibrium characteristics, geological and hydrological conditions, thermophysical parameters of geotechnical bodies, physical parameters, stratigraphic structure, distribution of aquifers, geothermal temperature, depth of thermal resources, geothermal properties, geothermal heat throughput, thermal conductivity, specific heat capacity, porosity
Economic evaluation	Investment estimates, operating cost estimates, revenue estimates, benefit assessments, net present value, project scale, operating cycle, heating and cooling prices, funding sources and interest rates, number of users and retention rates
Environmental impact	Atmospheric impacts, groundwater impacts, chemical contamination, subsidence collapse, etc., geothermal field equilibrium impacts, energy savings and emission reduction gains, heating and cooling season duration, thermal disturbances
Social security (pensions, medical insurance)	Government support policies, incentives, regulations and standards, public awareness, consumer confidence, technology reliability, efficiency, safety, technology diffusion, technological advancement

.

2.1.2. Optimization of Indicator Screening

As can be seen from the summary results from the analysis of the influencing factors, a large number of influencing factors were extracted from the four dimensions. There may be situations such as crossover and duplication among the factors, so it was necessary to screen and optimize the indicators. In this study we carried out expert consultation to determine whether the influencing factors are representative and scientific and whether quantifiable evaluation factors exist.

For the expert consultations, we spoke with experts from universities, research institutes, and enterprises with rich practical experience in the project. A total of five experts were invited to participate: one professor, two full senior engineers, and two deputy senior engineers. All of the experts are involved in shallow geothermal energy–related specialties such as geological exploration, engineering economics, air conditioning, and HVAC. The opinions of experts on evaluation indices and evaluation factors are shown in

Table 2. Expert advice.

Secondary Indicators	Evaluation Factor	Expert Opinion
Geological condition	Percentage of crushing belts	The fracture zone percentage is not a necessary item for the survey and is not well quantified; the current geological drilling ability of the area is evaluated in terms of rock hardness and is recommended for replacement.
Heat transfer performance	Unit length heat exchange	The heat exchange per unit of extended meter in winter and summer is greatly influenced by the ground temperature, which is usually opposite in winter and summer; where the ground temperature is low, the heat exchange is strong in summer and weak in winter, and vice versa: the heat exchange is weak in summer and strong in winter. Evaluation indicators should be differentiated between areas with different needs.
Project scale	Planned energy supply area	The project scale is recommended to be adjusted from the resource endowment dimension to an indicator of the economic evaluation dimension; the project Scale is not as large as it could be given the constraints. The area of buried pipes can be the most direct prerequisite for project implementation.
Benefit estimation	Net present value ratio	To comprehensively evaluate the investment feasibility of the project, it is recommended that common indicators such as payback period and financial net present value be used.
Energy price	Quarterly heating prices	Quarterly charges as a judgment standard are not appropriate; it is recommended to evaluate monthly charges. Evaluation should distinguish between different types of buildings; for example, the same price could be excellent for residential homes and poor for hospitals. Long heating and cooling times are poor, while a short heating time is good.
Incentive support policies	Number of policies	The main economic measurement difficulty of the project is the difficulty in making decisions regarding the land, while the price of electricity is the most important project cost. The return on investment has a large impact. The proposed factors influence the electricity price concessions > tax incentives.
Technological advancement	Investment cost decrease rate	Technological advances affecting investment and operating costs are difficult to quantify and are not meaningful for decision-making regarding current projects.

.

2.1.3. Evaluation Indicator System and Evaluation Criteria

According to the screening and adjustment of the indicators by the experts, the evaluation system of the development potential of shallow geothermal energy heating and cooling (vertical buried pipe heat exchange) projects in southern China is shown in Figure 1. The whole evaluation system contains four first-level indicators and eighteen second-level indicators.

On the basis of the identification of 18 secondary indicators, 18 evaluation factors were identified in terms of representativeness and quantifiability, and specific quantitative scoring criteria were defined, as shown in

Table 3. Description of scoring criteria and correlation of evaluation factors under secondary indicators.

Secondary Indicator	Evaluation Factor	Different Kinds	Score (of Student’s Work)	Grading Criteria	Description of Relevance
Geological Condition	Rock integrity degree	excellent	10	incomplete	Positive correlation Rock integrity consists of the degree of rock integrity, fracture zones, and karst development; the more complete the geological drill ability, the better.
		very much	7	more complete
		general	4	more broken
		differ from	1	very broken
Groundwater Conditions	Aquifer thickness [30] (m)	excellent	10	≥30	Positive correlation The total thickness of the aquifer within 200 m of the ground is an indicator of the suitability of the vertical buried pipe heat exchange system for zoning; the higher the thickness, the more appropriate.
		very much	7	30–20
		general	4	20–10
		differ from	1	<10
Geothermal Physical Parameters	Thermal conductivity [40] (W/m-k)	excellent	10	≥3.5	Positive correlation Geotechnical thermal conductivity determines the efficiency and stability of the ground source heat pump system; the higher the better.
		very much	7	3.5–2.5
		general	4	2.5–1.5
		differ from	1	<1.5
Geothermal Reservoir	Specific heat capacity [40] (1 × 106 J/m3 k)	excellent	10	≥2.5	Positive correlation The higher the specific heat capacity, the higher the amount of heat stored in the geotechnical body and the richer the geothermal resource.
		very much	7	2.5–2
		general	4	2–1.5
		differ from	1	<1.5
Heat Transfer Performance	Unit length of heat exchange [29] (W/m)	excellent	10	≥50	Positive correlation The amount of heat exchanged per unit of extended meter affects design parameters such as the number of wells drilled and the length of piping, which in turn affects the cost of construction; the higher the heat exchanger, the more efficient the system.
		very much	7	50–45
		general	4	45–40
		differ from	1	<40
Project Scale	Available energy area (1 × 104 m2)	excellent	10	≥100	Positive correlation The larger the area available for energy, the higher the potentially chargeable energy supply fee.
		very much	7	100–10
		general	4	10–5
		differ from	1	<5
Operating Cycle	Length of concession period [35] (years)	excellent	10	≥30	Positive correlation The longer the concession period, the longer the number of billable years.
		very much	7	30–25
		general	4	25–20
		differ from	1	<20
Capital Cost	Borrowing rate [37]	excellent	10	<3%	Negative correlation The higher the borrowing rate, the higher the cost of capital and the lower the program benefits.
		very much	7	3–6%
		general	4	6–10%
		differ from	1	≥10%
Price of Energy Supply	Monthly unit price of heating [36] (yuan/m2/month)	excellent	10	≥10	Positive correlation The higher the monthly unit price of heating, the more income you get from heating the same area.
		very much	7	10–8
		general	4	8–6
		differ from	1	Poor: <6
Estimated Charging Rate	User rate	excellent	10	≥85%	Positive correlation The higher the user usage, the more square footage is charged and the more revenue the program generates.
		very much	7	85–70%
		general	4	70–55%
		differ from	1	<55%
Benefit Estimation	Net present value [24] (dollars)	excellent	10	No incentive support policies > 0	Positive correlation NPV greater than 0 is the basis for making investment decisions; the higher the value, the higher the potential of the project.
		very much	7	Local policy + ≥ 0
		general	4	Simulation policy + > 0
		differ from	1	Simulation policy + < 0
Climate Weather	Duration of hot and cold seasons [40] (months)	excellent	10	≥8	Positive correlation The longer the hot and cold seasons, the longer the heating and cooling demand hours, the more efficiently the heat pump system operates, the more it charges, and the more economically efficient it is.
		very much	7	8–6
		general	4	6–4
		differ from	1	<4
Atmospheric Emission	Combined energy savings rate [20,30]	excellent	10	≥50%	Positive correlation The better the atmospheric emission reductions, the higher the potential for shallow geothermal heating and cooling development.
		very much	7	50–30%
		general	4	30–10%
		differ from	1	<10%
Geological Risk	Number at risk [20]	excellent	10	0	Negative correlation The lower the geological risk associated with shallow geothermal energy development, the higher the project safety and feasibility.
		very much	7	1
		general	4	2
		differ from	1	≥3
Geothermal Field Equilibrium Equalization	Winter heat uptake and summer heat release equilibrium rate [20]	excellent	10	≥90%	Positive correlation The more stable and balanced the geothermal field equilibrium is, the more efficient and sustainable the shallow geothermal energy development is.
		very much	7	90–80%
		general	4	80–70%
		differ from	1	<70%
Incentive Support Policies Deal	Number of policies [36] (subsidies, electricity tariff concessions, tax incentives, carbon emission reduction gains)	excellent	10	≥ Subsidies + tariffs	Positive correlation The more incentives and supportive policies, the higher the estimated project benefits.
		very much	7	≥ Subsidized + no tariffs
		general	4	≥ Unsubsidized + electricity prices
		differ from	1	No subsidies + no tariffs
Public Awareness	Shallow geothermal familiarity [43]	excellent	10	≥50%	Positive correlation Public awareness influences support for new energy products.
		very much	7	50–30%
		general	4	30–10%
		differ from	1	<10%
Technological Advancement	Investment cost decline rate [36] (5 years)	excellent	10	≥10%	Positive correlation Technological advances may lead to lower investment costs and management and operating costs.
		very much	7	10–5%
		general	4	5%–0
		differ from	1	<0

.

The evaluation factors and scoring criteria in the table were used for project evaluation after confirmation of indicator weights, and the specific method used was to obtain the corresponding evaluation grade and score according to the actual parameters of the evaluation factors of the project to be evaluated and then calculate the comprehensive score of the project according to the weight of the indicators. Then, investment decision-making suggestions were made according to the comprehensive score. The scoring division criteria and investment decision-making suggestions were as follows:

(1)

A score greater than or equal to 8 is a high-quality project with great development potential, which can be prioritized for investment decisions;

(2)

A total score of between 6 and 8 is considered a good investment project, with a high potential for development, and an investment decision can be made;

(3)

A total score of between 4 and 6 is considered a general project with some development potential, which requires further careful consideration before making an investment decision;

(4)

A total score of 4 or less is considered a failing item and does not support making an investment decision.

2.2. Calculation of Weights

2.2.1. Expert Ratings of Indicator Importance

Hierarchical analysis was used to determine the weight of each indicator in the indicator system, starting from the first-level indicators, using a 1–9 scale to compare two indicators at the same level and determine the relative importance. The same process was repeated with the second-level indicators to construct a judgment matrix. The 1–9 scale is shown in

Table 4. Meaning and description of the values of the scale method.

Relative Importance	Define	Display Format
1	equal importance	Horizontal versus vertical indicators
3	slightly important	Horizontal versus vertical indicators
5	important	Horizontal versus vertical indicators
7	high priority	Horizontal versus vertical indicators
9	vital	Horizontal versus vertical indicators
2, 4, 6, 8	midpoint	Horizontal versus vertical indicators
from the bottom (lines on a page)	insignificant	Horizontal versus vertical indicators

.

Recoverable geothermal reserves in geothermal reserves are the focus of shallow geothermal energy exploration and evaluation. The aim of this study was to choose geothermal reserves based on the evaluation index and resource endowment. Within the range that can be exploited by the current technology, the higher the value of specific heat capacity for the same development area and depth of different projects, the better the geothermal reserves and performance, the better it can meet the project demand and sustainable development, and therefore the better the evaluation result [50,51].

A total of 20 experts were invited to score the importance of the indicators in this session, including experts from: shallow geothermal development companies in the southern provinces of China, geological brigades of the Geological and Mining Bureau, research institutes, universities, and other relevant units. Thirteen of the experts had senior titles, and fourteen had more than 5 years of work experience related to shallow geothermal energy. All of them worked in the fields of heating, ventilation, air-conditioning, geological surveying, engineering economics, and other fields related to shallow geothermal energy.

2.2.2. Weight Calculation Based on Yeah Software

When the traditional hierarchical analysis method is used for multi-expert evaluation decision-making, there are difficulties in judgment matrix construction, error-prone computation, cumbersome consistency adjustment, etc. Yeah software can automate and visualize consistency adjustment and weight calculation and is widely used in benefit quantification [52], risk assessment [53], evaluation system exploration [54], and other fields. Therefore, in this study we used Yeah software V12.12 for judgment matrix construction and consistency tests and to calculate indicator weights.

In this study, we assembled calculation results. First, we carried out individual consistency tests for each expert’s judgment matrix and corrected unreasonable parts of the judgment matrix through the maximum improvement method [55] to obtained the sorting weights of each judgment matrix. Then, data assembly was completed using the geometric mean for the sorting weights of all the experts. Finally, the sorting weights of the assembled judgment matrix were used to compute the total sorting weights. The total ranking weights were calculated using the ranking weights of the assembled judgment matrices.

When the traditional hierarchical analysis method is used for multi-expert evaluation decision-making, there are difficulties in judgment matrix construction, large calculation volume, cumbersome consistency adjustment, etc. Yaahp software can automate and visualize consistency adjustment and weight calculation of the hierarchical analysis method and is widely used in benefit quantification [52], risk assessment [53], evaluation system exploration [54], and other fields. Therefore, in this study we used Yaahp-based software for judgment matrix construction and consistency tests and to calculate indicator weights.

(1) Calculation methods and principles

Yaahp software was used to take the average value of all expert judgments of the same element, form a comprehensive judgment matrix, calculate the weight ranking, and assemble the results. The calculation results assembly method retains the independent opinions of individual experts, so we used the calculation results assembly method, as follows.

Judgment Matrix Inputs and Consistency Tests

The results of the indicator scoring by the k experts were entered into the software with the name or number of the expert, as shown in Equation (1):

$$A^{(k)}=\left[a_{ij}^{(k)}\right]$$

(1)

A(k) is the judgment matrix of the kth expert and $a_{ij}^{(k)}$ is the two-by-two indicator comparison score in the judgment matrix of the kth expert. The judgment matrix of each expert needs to satisfy the positive and negative inverse of Equation (2):

$$a_{ij}^{\left(k\right)}{a}_{ji}^{\left(k\right)}=1 and a_{ii}^{\left(k\right)}=1$$

(2)

The corresponding eigenvector W was obtained by calculating the relative weight of each indicator in the matrix, as shown in Equation (3):

$$W^{(k)}={\left[w_1,w_{2,}\dots ,w_n\right]}^T$$

(3)

Then, we calculated the maximum eigenvalue of the judgment matrix ${\lambda }_{\max }$ and the CR value for the judgment matrix consistency test, as shown in Equations (4) and (5):

$${\lambda }_{\max }^{(k)}={\displaystyle \frac{A^{(k)}{w}^{(k)}}{w^{(k)}}}\left(k=1,2,\dots ,20\right)$$

(4)

$$C{R}^{(k)}={\displaystyle \frac{{{\displaystyle \lambda }}_{\max }^{(k)}-\mathrm{n}}{RI(n)·(n-1)}}$$

(5)

The CR(k) value should be ≤0.1, otherwise the matrix elements need to be corrected until consistency is satisfied. The consistency correction was performed using the maximum improvement method, i.e., adjustment of the minimization matrix was achieved by finding the value of the maximum deviation for correction.

Calculation of single expert weights

The eigenvectors were normalized so that the sum of all weights was equal to 1, which was calculated as shown in Equation (6):

$$w_i^{(k)}={\displaystyle \frac{w_i^{(k)}}{{\displaystyle {\sum }_{j=1}^n{w}_i^{(k)}}}}(i=1,2,\dots ,n)$$

(6)

The final k-independent weight vectors were obtained as shown in Equation (7):

$$w^{(k)}={\left[w_1^{(k)},w_2^{(k)},w_3^{(k)},\dots ,w_n^{(k)}\right]}^T$$

(7)

Aggregation of weighting results

The group decision-making approach used expert averaging of weights, and the output approach selected a geometric mean set of computed results, which averaged the k weight vectors by indicator dimension, as shown in Equation (8):

$${\overline{w}}_i=\sqrt[k]{{\displaystyle {\prod }_1^k{w}_i^{(k)}}}(i=1,2,\dots ,n)$$

(8)

${\mathrm{w}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{e} \overline{w}}_{\mathrm{i}}$ is the geometric mean weight of the ith indicator of the indicator among all experts, which was then normalized to obtain the final weight of the indicator, as shown in Equation (9):

$$w_i^{final}={\displaystyle \frac{\overline{w_i}}{{\displaystyle {\sum }_{j=1}^n{\overline{w}}_j}}}(i=1,2,\dots ,n)$$

(9)

3. Results and Discussion

3.1. Calculation of Weights and Analysis of Results

The judgment matrix formed by the scoring results of the 20 experts was input into Yeah software after judgment indicator correction, and finally obtaining the judgment matrix of the first-level indicators and the weighting results, as shown in

Table 5. Judgment matrix and weighting table for Tier 1 indicators (CR = 0.0753).

Development Potential A	B1	B2	B3	B4	Wi
Resource endowment B1	1	1.3909	2.6976	2.2953	0.396
Economic evaluation B2	0.719	1	1.9395	1.6502	0.2847
Environmental impact B3	0.3707	0.5156	1	0.8509	0.1468
Social support B4	0.4357	0.606	1.1753	1	0.1725

.

From the results of the comprehensive weight ranking of the first-level indicator weights of the four evaluation dimensions shown in Figure 2 and Figure 3, resource endowment (0.3960) > economic evaluation (0.2847) > social support (0.1725) > environmental impact (0.1468). Resource endowment was the indicator with the highest weight, which is derived from the fact that the abundance and extractability of shallow geothermal resources are the basis for the project to be able to be implemented and operated in the long-term, and therefore it occupies the absolute core position in the evaluation system. The economic evaluation, as the second weighted indicator, involves the project investment cost, operation cost, energy supply price, and benefit estimation, which are secondary economic indicators that directly reflect the project cost and benefit, and therefore occupy an important position in the comprehensive evaluation system. Although the weight of social support is relatively low, this indicator mainly reflects the support of the local government, public acceptance technological progress, etc. The adequacy of social support has an important impact on the sustainable development of the project and should not be ignored. The environmental impact, as the indicator with the lowest weight, indicates that environmental factors are of secondary concern in the current evaluation of the development potential of shallow geothermal projects. This result may stem from the fact that shallow geothermal heating and cooling applications are currently being paid attention to and promoted because of their green attributes, and therefore their environmental value is often taken for granted and easily ignored in the evaluation process. Since the development of shallow geothermal energy is essentially closely related to environmental protection and sustainable resource utilization, this attribute may be regarded as a basic condition in the design of the evaluation system, resulting in its weight being reduced. However, this weighting may also reflect that the current evaluation system fails to recognize the potential impact of environmental factors throughout the development cycle, or that it relies too heavily on the “green” labeling of shallow geothermal energy in the evaluation process and neglects to consider its potential environmental costs in detail. Therefore, although environmental impacts were given a secondary focus in the current evaluations, their impact on the sustainable development of shallow geothermal projects needs to be further investigated to ensure a comprehensive and objective assessment of their development potential.

The secondary indicators ranked in the top four and with a weight value greater than 0.08 included heat transfer performance (0.0952), incentive support policy (0.0872), geotechnical thermophysical parameters (0.0823), and groundwater conditions (0.0815). Other than incentives and support policies, the rest of the indicators belong to the resource endowment evaluation dimension, which highlights that the resource abundance and availability of shallow geothermal energy development is the primary prerequisite and foundation for project implementation. It is worth noting that the incentive support policy is the only secondary indicator in the top four among the other evaluation dimensions, which indicates that the government’s policy support is crucial to the evaluation of the potential of shallow geothermal projects at this stage, and the policy factor plays an indispensable role in promoting the project development. In addition, other important indicators with weight values greater than 0.05 included geothermal reserves (0.0727), geological conditions (0.0643), benefit estimation (0.0623), and price of energy supply (0.0514), which directly reflect the feasibility and economic value of shallow geothermal energy development. In contrast, indicators that indirectly affect the feasibility of the project, such as public perception (0.0372), had relatively small weights, especially atmospheric emissions (0.0289), which was the lowest among all indicators, suggesting that the current evaluation system may have underestimated the economic and social value of shallow geothermal heating and cooling technology in terms of energy saving and emission reduction, or that its environmental benefits are sufficiently significant to be fully taken into account. This suggests that the current evaluation system may underestimate the economic and social value of shallow geothermal heating and cooling technology in terms of energy saving and emission reduction, or that its environmental benefits are significant enough and thus not fully considered.

Overall, the results of this weight ranking reflect that the current evaluation system attaches great importance to geothermal resource conditions, policy support, and economic factors, and at the same time reveals insufficient attention to some indirect impact factors. Such an allocation of evaluation weights reflects, to a certain extent, the real needs of shallow geothermal energy development, but at the same time may also ignore the potential environmental and social impacts.

3.2. Case Studies

An X ground source heat pump central air-conditioning project in Guizhou was completed and put into use in 2016 and has been running for nearly 8 years. Some of the project’s survey, design, and operation data using the quantitative evaluation system were obtained for case analysis, and the evaluation results and evaluation scores are shown in

Table 6. Case analysis results.

Estimation Norm	Indicator Weight	Evaluation Factor	Project X Data	Case (Law) Grading	Case (Law) Score
C1	0.0643	Rock solidity	More complete	very much	7
C2	0.0815	Aquifer thickness	25	very much	7
C3	0.0823	Thermal conductivity	2.286	general	4
C4	0.0727	Specific heat capacity	1.2	differ from	1
C5	0.0952	Heat exchange per unit of linear meter	56.242	excellent	10
C6	0.0433	Usable area	105,333.8	very much	7
C7	0.0401	Concession period	30	excellent	10
C8	0.0491	Interest rate on borrowing and lending	0	excellent	10
C9	0.0514	The monthly unit price of heating	11	excellent	10
C10	0.0385	The utilization rate of users	80%	very much	7
C11	0.0623	Net present value (NPV)	Simulation policy > 0	general	4
C12	0.0403	Duration of the summer and winter seasons	7	very much	7
C13	0.0289	Comprehensive energy-saving rate	30%	very much	7
C14	0.0365	Number of risks	0	excellent	10
C15	0.0411	Winter heat absorption and summer heat release equilibrium rate	97%	excellent	10
C16	0.0872	Number of policies	0	differ from	1
C17	0.0372	Shallow geothermal familiarity	20%	general	4
C18	0.0481	Investment cost reduction ratio	2	general	4
				Totals	6.2911

.

The total evaluation score of the case project was 6.2911, which is a good investment project according to the division criteria, with a large potential for development and support for making investment decisions. However, the project from a single indicator, the number of incentives and supportive policies is 0, based on which the financial net present value of the commonly used economic evaluation indicators is less than 0, which does not support the investment decision, contrary to the investment decision-making recommendations of the evalua-tion system.

From the point of view of the actual operation of Project X [56]: (1) the utilization rate from the first year was about 30%, fluctuating to as high as 80%, and the project revenue gradually exceeded the expenditure, resulting in stable income; (2) the project’s ground-source heat pump air-conditioning system resulted in annual energy and cost savings of 57%, so the economic benefits were significant; (3) the actual running time of the air conditioning system was more than 8 months per year, which achieved balanced cooling and heating while being more economical and environmentally friendly, saving about 2520 tons of coal and reducing carbon dioxide emissions by about 6405 tons per year, which is a remarkable energy-saving and environmentally friendly effect; (4) the project not only provided additional jobs such as operation and maintenance of the system, alleviating the pressure on the society for employment and cultivating professionals in the field of HVAC, but also passed acceptance as a demonstration project for the application of renewable energy building in Guizhou Province. The project was also accepted as a Guizhou Province Renewable Energy Building Application Demonstration Project and selected as one of the “National Compendium of Typical Cases of Renewable Energy Heating”, which aroused a favorable response from society.

The quantitative evaluation system developed in this study gave investment advice that was consistent with the operation of the project, including four evaluation dimensions and eighteen secondary indicators of the evaluation system, instead of a single financial evaluation index, and thus yielded a more comprehensive reference value.

4. Conclusions

This study identified key factors from the perspective of the whole lifecycle of shallow geothermal energy development and utilization, optimized indicators and scoring standards after expert consultation, constructed an evaluation system for the development potential of southern shallow geothermal energy heating and cooling projects containing four first-level indices and eighteen second-level indices, and provided a quantitative evaluation method using the hierarchical analysis method.

The current development of shallow geothermal energy projects in southern China is mainly driven by policy, often lacking comprehensive consideration of market and other factors. In addition, there is a lack of reliable assessment tools for the development of marketable and commercial projects. The development and utilization assessment model in this paper solved this problem. Validation of the methodology requires more project data for case study analysis. Comparison of the case study results with the actual operation results is an important future direction. The main conclusions from this study are as follows.

(1)

Among the primary indicators, resource endowment (0.396) and economic evaluation (0.2847) had the largest weights, indicating that the availability of shallow geothermal resources is a prerequisite for development, and economic evaluation is an important basis for investment decisions. Among the secondary indicators, heat exchange performance (0.0952), incentive support policy (0.0872), geotechnical thermal properties (0.0823), and groundwater conditions (0.0815) had prominent weights, implying that project development, in addition to considering resource suitability, also needs to pay attention to policy support.

(2)

The case study showed that the development potential evaluation results were consistent with the actual operation of the project, contrary to the decision of the financial NPV single index, which shows that the comprehensive reference value of the system is higher. The quantitative evaluation system for the development potential of shallow geothermal energy heating and cooling projects in the southern region constructed in this paper needs to be further verified.

Recommendations

The following recommendations are made for the development and utilization of shallow geothermal energy in southern China:

(1)

Detailed investigation of resource endowment, resource endowment of shallow geothermal energy development, and utilization assessment are very critical, such as heat transfer performance, geotechnical thermophysical parameters, and other indicators. In view of the large differences in topography and geomorphology in the southern re-gion, should be in the survey area and specific indicators of resource endowment con-ditions survey for refinement.

(2)

When evaluating and examining geothermal projects, subsequent consideration could be given to incorporating market uncertainty factors. User utilization rate has a large impact on the benefit assessment of shallow geothermal energy heating and cooling projects. Project assessment and economic evaluation in the southern region should incorporate user utilization rates, which can help to accurately formulate government incentive policies and reduce investor decision-making risks.

(3)

Consider synergizing project scale with supportive policies when planning the development of geothermal projects. Direct subsidies, tax incentives, and CCERs are key policies to support the development of shallow geothermal energy, but a single policy is insufficient to support investment decisions. Governments need to adopt a diversified policy strategy to raise project investment thresholds and expand project scales.